| 摘要 |
In a subscriber line interface circuit (1) connected to a two-wire telephone line(A, B) for transmitting AC signals, one (A) of the wires is set to a high DC voltage lower than a first DC voltage (GND), while the other wire (B) is set to a low DC voltage higher than a second DC voltage (VREG). First means (10, 11) are provided for setting, for AC signals on said one wire (A), a first threshold voltage (VTHA) between said first DC voltage (GND) and said high DC voltage, and/or, for AC signals on said other wire (B), a second threshold voltage (VTHB) between said low DC voltage and said second DC voltage (VREG). To automatically adapt the signal headroom to the actual amplitude of the AC signals, second means (2, 3, 4, 5) are provided for controlling the voltage difference (VA) between said first DC voltage (GND) and said high DC voltage and/or the voltage difference (VB) between said low DC voltage and said second DC voltage (VREG) as long as the instantaneous value (va) of the signal on said one wire (A) equals the first threshold voltage (VTHA) and/or the instantaneous value (vb) of the signal on said other wire (B) equals the second threshold voltage (VTHB).
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